CIRCUIT BREAKER OPERATOR SIGNATURE ANALYSIS

Wesley R. SpeedTXU Electric USA

SUMMARY

Over the years utilities have experienced problems with slow operation, failure to tripor failure to close of circuit breakers. In recent years many of these problems have beentraced to faulty lubrication of the circuit breaker operating mechanisms. While traditionaldiagnostic testing procedures are useful in determining the condition of the breakerinsulation, contact resistance and operating time, they are not adequate to fully evaluatethe condition of the breaker operating mechanism. The concept of measuring the controlcircuit characteristics of the circuit breaker during operation to evaluate breaker operatorperformance has been around for some time, but only in recent years has it becomeavailable in a lightweight, portable package that allows quick capture of data and analysisof results. TXU Electric has found circuit breaker operator signature analysis to be anextremely beneficial tool for evaluating the condition of breaker operating mechanisms,prioritizing the maintenance of circuit breakers, and minimizing equipment outages.

BACKGROUND

For many years electric utilities have relied on traditional diagnostic testingprocedures (power factor, megger and time) to evaluate the condition of their circuitbreakers. These tests provide information concerning the integrity of a breakersinsulation system, contact resistance and main contact operating times. However, theseprocedures do not give meaningful insight into the condition of the operating mechanismexcept for overall speed. Since a large percentage of breaker problems can be attributedto the operator and related components, we were in effect looking for problems in thewrong places.

TXU Electric has experienced situations in which a breaker is suspected to be slow(from a DFR trace or when a transformer is outaged on overload). Timing tests are thenperformed on the breaker, but it shows to be within specifications. At this point we haveblamed the relaying system or other culprits. Another situation is a breaker that burns upa trip coil during a fault, and we blame a faulty trip coil.

Situations such as these may indicate a failure of the circuit breaker operatingmechanism lubrication. Circuit breaker operators contain numerous mechanicalcomponents that rely on proper lubrication for their successful operation. However,many enemies of the lubrication, including age, temperature, airborne contaminates andimproper or infrequent maintenance practices have left many circuit breaker operatorsprone to failure.

The Lubrication Guide of the Doble Circuit Breaker Committee [1] discusses theseissues of lubrication and maintenance practices at length. An excerpt is taken from thisguide as follows:

Transmission and distribution breakers have become a maintenance challengebecause of their extended time in the field without relubrication Circuitbreaker failures have been traced to faulty lubricants and/or questionablelubrication practices. Many circuit breakers in use today are very old and continueto be lubricated with what the manufacturer specified many years ago. Many ofthese lubricants will not last. Some greases will separate, leaving only a drythickener which can slow the breaker action. Some greases and penetrating oilscan change in physical form, leaving what appears to be a varnish-like residue inbearings and other critical friction areas. [1]

The Lubrication Guide of the Doble Circuit Breaker Committee [1] also goes on toexplain how the use of synthetic greases in lieu of petroleum based greases shouldprovide longer service life of the breaker operator between lubrications.

Since traditional diagnostic testing procedures fall short of providing thorough dataregarding the breaker operating mechanism, another means is necessary to provide thisinformation.

Old Idea, New Package

The idea of measuring a breakers control circuit electrical information duringoperation to provide insight into the condition of the operating mechanism has beenaround for many years [2]. Graphical curves containing this electrical information can beplotted and analyzed. The electrical trace for a trip or close operation of a circuit breakercan indicate the health of the breaker operator. This shot can also be compared to othershots of the circuit breaker in order to track its performance over time. Operatingmechanism problems are evidenced in changes in the characteristics of the electricaltraces. In addition, shots of a breaker can be overlayed with shots of breakers with likeoperator types for comparison.

Previous technologies used to implement this concept required hard wiring ofconnection equipment into the circuit breaker. While the cost of equipment andinstallation was justifiable for breakers located at extremely critical system locations, itunfortunately became cost prohibitive to implement on a larger scale.

However, in recent years, lightweight, portable technology has become commerciallyavailable which does not require hard wiring to the breaker. Connection to the breakercontrol circuitry can occur safely while the breaker is still in service, and the wholeprocess of testing the breaker only takes a few minutes. Information is immediatelyavailable from the portable device, and the data obtained can be downloaded to acomputer for future in-depth analysis.

Monitored items include secondary CT currents, trip and close coil current, and DCvoltage supply to the breaker. Analysis of the data provides insight into the overallcondition of the breakers operating mechanism and lubrication, main contact operatingtime, trip and close coils, auxiliary switches, DC voltage supply and control circuitwiring.

One such product currently available which accomplishes the above is the KelmanProfile P1 CB Analyzer. TXU Electric purchased this product and initiated a program totest system circuit breakers. The remainder of this paper discusses the experiences gainedfrom utilizing this piece of test equipment.

IMPLEMENTATION

Connection

The connection of the breaker analyzer is extremely simple, consisting only of fourleads, as indicated the Figure 1. First, two leads are attached to the DC voltage supply tothe circuit breaker. Then, an AC clamp-on is connected to the secondary of one phase ofthe current transformers (providing main contact operating time). Finally, a DC clamp-onis connected to the trip and/or close circuit (providing DC current amplitude over time).

Figure 1 Connection of Circuit Breaker Analyzer

A typical shot obtained from the breaker analyzer is shown in Figure 2. There arethree main features the main contact time, the DC supply voltage, and the trip / closecoil current during the operation of the breaker. How the mechanical movement of theoperator corresponds to the electrical trace is indicated in the Figure 2. When a trip signalis initiated to the breaker, the DC current through the trip coil starts to rise, moving theplunger inside the coil. The plunger moves until its hits the trip latch of the breaker. Thisis indicated on the electrical trace by the valley after the first hump on the curve. Up tothis point, the breaker is stationary. For a proper breaker operation, the trip latch is

released, the energy in the stored energy mechanism starts in motion, and the breakercontacts start to move. At some point the main contacts break the current through thebreaker, and an auxiliary contact opens and breaks the current in the trip circuit.Meanwhile, the DC supply voltage is measured during the shot.

Figure 2 Typical Trip Shot

Initial Program

For our initial program we tested 15kV, 25kV and 69kV circuit breakers. Thesevoltage classes were selected because breakers of these voltages historically caused us themost problems. In phase 1 of this program, we were successful in identifying breakers infailure mode, and corrective action was taken. We feel that numerous equipment outageswere avoided as a result of performing breaker analyzer testing. About six months laterwe initiated phase 2 of the program. During this time we refined our data collection andstorage techniques, utilized the data to prioritize maintenance, documented healthy

Main Contact Time

a Contact Opens

Trip Coil SolenoidHits Latch

Breaker Comes off Latch,Operator Starts to Move

DC Supplyto Breaker

breaker operators, identified bad actors by breaker type, and expanded our programs toinclude 138kV circuit breakers.

Examples of problems found while using the breaker analyzer were failed operatingmechanism lubrication, damaged trip and close coils, dirty auxiliary switches, looseconnections in the control circuitry, substation battery or battery charger problems,improper control cable sizing and tailsprings out of adjustment. In addition, while triptesting these breakers we found problems with reclosing relays, breaker closing motors,and RTUs.

To perform the testing, we assigned two two-man teams who focused exclusively onbreaker analyzer testing. Each team consisted of a person to run the breaker analyzer anda patrolman to perform the switching. The process of connecting the breaker analyzer tothe breaker and running the tests took only about 5 minutes, plus time to switch theequipment out. Average testing, switching and travel time for each team was around 23minutes per breaker. They averaged over 20 breakers per day, accomplishing testing onas many 50 breakers in a given day. It took these two teams about two months to test over700 circuit breakers.

Shot Analysis

The process of analyzing the shots is a somewhat tedious process, but well worth thetime considering the outages that can be prevented from finding problems. A process thatseemed to work well was to first pull up trip shots for a particular breaker utilizing thebreaker analyzer software. We would inspect the shots to see if they fell within thefollowing general measurement criteria:

Main Contacts < 50ms on trip shot (3 cycle breaker) Main Contacts < 200ms on close shot Breaker off latch < 17ms (1 cycle for a 3 cycle breaker) Voltage drop < 10%

Then we looked to see if the second shot was faster than the first. This is a goodindication that the breaker was sluggish on the first shot due to faulty lubrication. Thebreaker would trip faster on the second shot since the breaker had been exercised andlimbered up. This provided a good graphical representation (See Figure 3) of why in thepast we would have a suspected slow breaker, trip the breaker to take it out of service toconnect the timing equipment, and the breaker timed within specifications. In taking thebreaker out of service, we were missing the all-important first trip. By using the breakeranalyzer, we now have a means to capture the first trip.

After the breaker was compared to itself, we compared its shots to other breakers ofthe same operator type. These families of curves could be compared against each other todetermine a prioritization of which breakers deserved the most immediate attention (SeeFigure 4). We would also compare a breaker to one in which we had completelyoverhauled the operating mechanism knowing it to be in the best possible shape. Shots

of each breaker are stored so that when future shots are taken we can compare them to theoriginal baseline of the breaker its signature to see if the operators performancedegrades over time.

Early in the program we focused on the breakers which showed large noticeabledifferences in their shots. These were our breakers in a critical failure mode. However,we began to realize the slight nuances between two shots from the same breaker could besignificant and should not be overlooked. In other words, shots from a healthy breakeroperator should have identical shots not only on their first and second trips, but on theirtrip shots compared over months or years of time.

An example of a breaker in which the DC supply voltage dropped more than 10% isshown in Figure 5. Other problems with the batteries or battery chargers could be pickedin the shots as a 60Hz ripple superimposed on the DC trip current signal (See Figure 6).In a couple of instances, the breaker analyzer shots indicated loose wires in the controlcircuitry (See Figure 7). Another item that can be identified with the breaker analyzer area tailspring out of adjustment (fast trip, slow close). Adjustment of the tailspring has beenused on occasion to speed up the trip on a slow circuit breaker, but this overlooks the rootcause of the slow breaker is probably faulty lubrication. Other items found were trip coilson the verge of failure and dirty auxiliary switch contacts.

Figure 3 Evidence of Faulty Lubrication in Breaker Operator

First shot, breakerslow to come off latch.

Second shot

Shot taken afteroperator overhaul.

Figure 4 Comparison of a Family of Curves

Figure 5 Evidence of Battery Problem

Three breakers of sametype, one with slowingtrip latch.

Drop in battery voltageduring shot.

Figure 6 Evidence of Battery Problem

Figure 7 Loose Connections in Control Circuit

Failed batteries, batterycharger doing the work.

Loose connectionsin control circuit.

Voltage trace

Current trace

Lessons Learned

We learned many lessons in the implementation of the circuit breaker signatureanalysis program. These are briefly described as follows:

Using the Breaker Analyzer

For the most meaningful analysis, it is critical that thought be placed into how thedata is managed. We purchased our breaker analyzers with a bar code feature thatproved to be extremely helpful in maintaining consistent data.

In order to obtain the best information about the breaker operator, it is critical to catchfirst shot. When a breaker with problems in the operator is not exercised in arelatively short period of time (as short as one month), the next breaker operation maybe slow and sluggish. Once the breaker has been exercised, it frees up to a point andas long as the breaker gets off of the latch, the stored energy will mask the bearingproblem temporarily. It is helpful to note the date of last previous operation on abreaker before using the breaker analyzer.

In order to achieve consistency in where you connect to the breaker each time, and tomake available the breaker analyzer testing duties to a broader range of employeeskill levels, we found it helpful to use tape to color code the placement of the fourleads within each breaker tested.

Most breaker operators would show a difference in speed between the first andsecond shots if the lubrication was faulty. However, a few of particular type operatorswould be consistent on the shot information, even thought their lubrication was in badshape.

The breaker analyzer we used was sensitive to connection of the DC voltage supplyleads to AC. It is a good practice to always check first with a voltmeter.

The breaker analyzer we used had some memory limitations which necessitateddownloading after completing tests on approximately twenty or thirty breakers.

Overhauling Breaker Operating Mechanisms

Different maintenance crews may have different definitions of operating mechanismoverhaul. In order to completely address the problems associated with faultybreaker lubrication, breaker operating mechanisms should be fully disassembled,cleaned, lubricated, reassembled and adjusted.

While a complete overhaul of an operating mechanism once seemed a daunting task,we have found that an experienced two man crew can generally perform a lowvoltage breaker overhaul within a day, and a high voltage breaker overhaul in twodays.

We found that in many cases the best practice is to replace all of the bearings whenperforming a breaker overhaul. Purchasing a full set of bearings for a breaker is oftencheaper than attempting to clean and relubricate in the field. A complete set ofbearings for a 15kV breaker is approximately $50, $300 for a 138kV breaker. Mostbearings are available over the counter, except a few specialty trip latch bearingsfound on particular 138kV breakers. New bearings can be cleaned, lubricated and

placed in sealed plastic bag for installation in the field. Close attention should be paidto load bearing bearings. These can have flat spots that can cause a breaker to bind upduring operation.

If a complete operator overhaul is being performed, consideration should be given tooverhauling all components associated with the breaker operation for instance pilotvalves on air systems.

As experience is gained in overhauling breaker operators, we found that it was easierfor some operator types to completely remove an operator from the breaker cabinetand overhaul it than to attempt to do it with the operator still in the breaker. Timesavings can be incurred if a spare operator can be located, overhauled, then used fordirect replacement in field.

Due to manpower limitations, it can be difficult to overhaul all breakers identified ina short period of time. However, it is important to take necessary steps to minimizethe chance of equipment outages. Since the trip latch is the weak link in the operationof the breaker, consider overhaul of the trip latch and light lubrication of the operatoruntil such time a complete overhaul can be performed.

General

The breaker analyzer does not perform all of the functions of a complete breakertimer (velocity, etc), but it does allow you to catch the first shot which a breakertimer can not do.

The consequences of faulty lubrication in vacuum breakers seem to be more severethan in oil breakers. The newer vacuum breakers require less stored energy to performthe breaker operation than older oil breakers did. However, the reduced stored energybecomes a disadvantage when bearing lubrication becomes faulty. Once a breaker isoff the trip latch, the older oil breakers have more energy to overcome bearingproblems than the vacuum breakers do.

Manufacturers are still specifying petroleum-based greases in their breakers, andwarranty considerations need to be taken into account when overhauling an operator.Hopefully manufacturers will consider utilizing synthetic greases in future.

There is benefit in a testing program in just being there. We found many problemsthat although they were not found by the breaker analyzer, they surfaced just becauseyou were doing testing.

The breaker analyzer is an excellent tool to help prevent customer outages. One disadvantage of the breaker analyzer we used is that it will not give a meaningful

current trace for AC-close circuit breakers. However, the information from the maincontact time and DC supply voltage to the breaker is captured.

Another disadvantage of the breaker analyzer is that it does not easily accommodatetesting of independent pole breakers.

When you are doing your breaker analyzer testing, you will invariably run intoproblems that need immediate attention. Therefore, it is advisable to have access tothe proper maintenance personnel during testing.

Trip testing is the best, cheapest maintenance you can do. Failure mode is not linear for all operator types.

CONCLUSION

Circuit breaker signature analysis has proved to be an excellent innovation forassessing the condition of circuit breaker operating mechanisms and related components.This is especially significant since a large percentage of equipment outages are attributedto circuit breakers. Circuit breaker signature analysis can also be utilized as an effectivetool to prioritize the work to be performed to identify the equipment that needs work,and also the equipment that does not need work. In the future, we plan to performperiodic breaker analyzer tests to see if there has been any deviation in the condition ofthe operator. We will also test any new circuit breakers that we install on the system toestablish a baseline signature for the operator. In addition, we hope to track theperformance of synthetic greases in the breakers that we have overhauled to track itsperformance in the field. We are not abandoning power factor, megger and timing tests ofcircuit breakers for they serve a different purpose. However, we believe that circuitbreaker signature analysis can be used as an excellent tool to assess the condition of ourbreaker operators, prioritize maintenance and minimize equipment outages.

REFERENCES

[1] Doble Circuit Breaker Committee, Lubrication Subcommittee: Lubrication Guideof the Doble Circuit Breaker Committee, 1995.

[2] G. K. Nelson, C. A. Zimmerman, Circuit Breaker Response Time Testing, AnEvaluation, 1991 Transmission & Substation Design & Operation Symposium,September 1991.

BIOGRAPHY

Wesley R. Speed is the Relay Support Manager at TXU Electric. Mr. Speed receiveda Bachelor of Science degree in Electrical Engineering at Texas A&M University in1990. He worked six years in engineering as a substation project engineer and three yearsin field operations before transferring to System Protection in 1999.